Aircraft main propulsion engines not only generate propulsion thrust for the aircraft, but in many instances may also be used to drive various other rotating components such as, for example, generators, compressors, and pumps, to thereby supply electrical, pneumatic, and/or hydraulic power. In multi-spool turbofan gas turbine engines, this is accomplished via a plurality of turbines. In particular, each turbine receives a flow of combusted gas from a combustor and, in response, generates shaft power that is used to drive one or more of these rotational components, via a plurality of concentric shafts. Thus, a portion of the shaft power that each turbine generates is used to provide propulsion thrust, while another portion of the generated shaft power is extracted and used to drive these other rotational components.
In recent years, aircraft architectures are being provided that rely more and more on electrical power, and less on pneumatic (e.g., bleed air) and/or hydraulic power. In such architectures, shaft power extraction demand can increase significantly. For example, shaft power extraction demand can be as much as 200% to 300% more than traditional solutions such as bleed air and/or hydraulic power. Although these architectures are generally safe, robust, and reliable, the architectures may suffer certain drawbacks. For example, high shaft power extraction demand can negatively impact engine compressor surge margin. A relatively straightforward technique to mitigate this drawback is to increase the engine speed and bleed during high shaft power extraction demands. This solution, however, can increase both the fuel burn rate and the residual thrust that the engine generates, most notably during flight and ground idle conditions, as well as during transients. The increased residual thrust can result in increased brake usage on the ground and/or increased airbrake usage in flight.
In order to meet the above-described needs of the more electric aircraft, architectures have been proposed that use the low pressure turbine to drive one or more generators. However, because the low pressure turbine operational speed range is, in many instances, significantly greater than the operational speed range of the high pressure turbine, the operating speed range from the low pressure turbine to an associated generator may need to be reduced. Moreover, it is further desirable that the low pressure turbine-driven generators, if included, are operable as starter-generators that can be used to provide starting torque to the gas turbine engine, if needed or desired.
Hence, there is a need for a system for a more electric aircraft architecture that improves the surge margin of the propulsion engine compressors and/or improves engine operability and/or, at least during high power extraction demands, reduces fuel burn rate and/or reduces the residual thrust that the engines generate, and/or reduces the operating speed range from the low pressure turbine to an associated generator and/or allows a starter-generator that is normally driven by the low pressure turbine to supply starting torque to the engine. The present invention addresses one or more of these needs.